Data centers are facilities that house servers and other types of computing equipment. For example, the servers that provide online service for a retailer, a bank, a school, etc., may be housed in a data center. Because of the nature of the equipment housed in data centers, and because of the types of services that this equipment is used to provide, the expectations for uptime at a data center are very high. While there may be occasional outages due to maintenance or software failure, it is generally expected that abrupt interruptions of electrical service to the equipment will not occur. In some cases, the equipment and the software running on the equipment are not designed to fail gracefully in the event of electrical service interruption. Rather, the data center is expected to provide continuous electrical power to the equipment so that failures due to electrical service interruptions do not occur.
In order to maintain continuous electrical service, data centers are generally equipped with backup electrical systems. These backup systems may include generators and Uninterruptable Power Supplies (UPS). In general, UPS's provide short-term power in the event of a utility power service interruption, in order to provide enough time to switch power over to the backup generators. Thus, for much of the duration of an extended utility power outage, power is provided by the generators. In theory, a data center may have enough generator capacity to replace completely the utility power that has been lost. However, equipping a data center with that amount of generator power is expensive, and thus may not be economically feasible in certain situations. Moreover, generator power is not exactly the same as the utility power that it replaces, since the types of generators that can be used on-site may be more susceptible to impedance issues than the utility power that the generators replace.